The present invention relates to an electrically conductive polyurethane elastomer and in particular to asymmetric ionic quarternary ammonium salts to extend the electrical life of polyurethane elastomers. It has particular application in electrostatographic printing apparatus and in particular as a biased transfer member in transferring toner from an electrostatographic imaging surface to a receiving surface such as a sheet of paper.
In the process of electrophotographic printing, a photoconductive surface is charged to a substantially uniform potential. The photoconductive surface is image wise exposed to record an electrostatic latent image corresponding to the informational areas of an original document being reproduced. This records an electrostatic latent image on the photoconductive surface corresponding to the informational areas contained within the original document. Thereafter, a developer material is transported into contact with the electrostatic latent image. Toner particles are attracted from the carrier granules of the developer material onto the latent image. The resultant toner powder image is then transferred from the photoconductive surface to a sheet of support material and permanently affixed thereto.
This process is well known and useful for light lens copying from an original and in printing applications from electronically generated or stored originals.
In a reproduction process of the type as described above, it is common practice today to use a biased transfer member to transfer the developed image from the photoconductor to the final support material such as a sheet of paper. Typically, these biased transfer members take the form of a roll and are comprised of a polyester based polyurethane with an additive to control resistivity such as tetraheptyl ammonium bromide. While capable of performing satisfactorily as a bias transfer member, in a transfer system wherein only about 25 microamps are applied to the transfer member difficulties are experienced in other systems, particularly, in multicolor imaging systems where more than one toner and a greater pile height of toner on the photoconductor must be transferred to the copy sheet. These systems require additional current up to 75 microamps. While the polyester based polyurethanes containing a tetraheptyl ammonium bromide operate satisfactorily at low current loads (25 microamps) having an electrical life of up to 1,500,000 prints at these increased current loads (75 microamps) we have found that the electrical life is shortened to approximately 500,000 prints. This is believed to be due to the depletion of the conductive additive in the polyurethane, in that the ionic components want to separate and over time the conductive additive is depleted, the resistance of the transfer member increases and under constant current the voltage on the power supply maximizes leading to a transfer system failure. This is believed to be due to the fact that the present systems are very low crosslinked systems and therefore the additives have high mobility and are very easily transported through the elastomer network, resulting in the diffusion of the charged components and accordingly reduction in conductivity of the transfer member. These prior art systems are described in U.S. Pat. No. 3,959,574 to Seanor et al. and U.S. Pat. No. 3,959,573 to Eddy et al.
Furthermore, attempts to use a new polyether based polyurethane exhibiting high wear resistance and low compression set as described in commonly assigned copending application Ser. No. 07/767,442 entitled "High Wear Resistance Low Compression Set Polyurethane" filed Sep. 30, 1991, in the name of Schlueter et al. as a biasable member with the tetraheptyl ammonium bromide additive met with failure in that the tetraheptyl ammonium bromide poisoned the catalyst so that the degree of cross linking was inadequate.
With respect to the polyether based elastomers described in the above referenced concurrently filed application, we have found that the higher the molecular weight of the polytetramethylene ether glycol the more difficult it is to clean a transfer member of machine debris, including toner, and accordingly the lower molecular weight polytetramethylene ether glycols are preferred in certain applications. For example, it takes more than four times as much air velocity (416 vs. 96 feet/second) to blow debris off an elastomer surface made with a polytetramethylene ether glycol having a molecular weight of about 2900 than one having a molecular weight of only about 650 and accordingly the lower molecular weight glycols are preferred. However, the curing of the elastomers made from the lower molecular weight glycols is exothermic raising the temperature to a level such that the conductive additives in the cross referenced concurrently filed application decompose. Accordingly, there is a need for an additive that provides stable conductivity, will not decompose during curing and is easy to clean.